"You can also see it's a pretty darn primitive brain. It would be small for a monkey or an ape," Simons adds.

"So it's telling us that the speed of achievement of brain enlargement in primates was a little slower than perhaps we had thought."

This skull of a small female was uncovered in a quarry southwest of Cairo in 2004. It was better preserved than another skull of a larger male of the species found in the same area in 1966.

Based on earlier finds, scientists had theorised the species had a relatively large brain.

Instead, it had a brain that might have been even smaller than that of a modern lemur, a primate with primitive traits.

The condition of the earlier skull prevented the analysis possible with the newer one.

Simons says that when this primate lived, Africa was an island, limiting the competition for survival.

He says brain enlargement may have evolved in this lineage after Africa became connected to Asia, bringing in more animals including new and dangerous predators.

"Brain-volume enlargement is favoured under conditions of competition because you need to be smarter," Simons says.

Dawn Ape

Aegyptopithecus is sometimes called Dawn Ape. Simons says it looks somewhat like an ape, particularly in its teeth and skull, and says it is thought to be close to the ancestry of monkeys, apes and humans.

"Because of the proportions of having a fairly robust chewing mechanism and a small brain, its skull looks like an ape's skull. It looks like a miniaturised gorilla," Simons says.

The new skull fits easily into a person's palm and is less than half the size of the 1966 skull.

The researchers think it was from a female weighing perhaps 2.5 kilograms, while the earlier one was from a male more than twice as big.

They say this size difference between the sexes of this species is similar to that of gorillas.

Simons says that he previously overestimated its brain size based on features of the 1966 skull, which has a bigger snout and more pronounced crests. Those features now seem to be attributes of a male of the species.

Other aspects of its remains indicate it was branching off from its lemur-like ancestors.

For example, its skull indicates the brain's visual cortex was large, suggesting it had very good vision, an important characteristic of higher primates.

Its eye sockets also indicate it was active during the day. Many more primitive primates are nocturnal.